In integrated services digital network ("ISDN") applications, a customer is interconnected with a communications network through a trunk line, such as a T1 trunk. Shown in FIG. 1, a T1 trunk 10 ("T1") contains 24 channels multiplexed according to time division multiplexing. The T1 uses out-of-band signaling: Of the 24 channels, a single "D" channel contains control signaling that is necessary for the customer to interact with the communications network. For example, call set up and tear down is accomplished via the D channel. The D channel occupies a specific time slot on the T1. The remaining 23 "B" channels contain voice, data or any kind of user communications data.
FIG. 1 illustrates the architecture of an end office switch 20 in known communication networks. An end office switch is a communication network switch connected directly to a customer. The customer's communication equipment, such as a telephone, computer, facsimile machine or a private branch exchange, interfaces with the end office switch 20. The end office switch 20 is the first of what may be several switches that route calls through the network.
T1's also interconnect switches within a communication network. In long distance telecommunication services, a call may be routed by several switches to connect the two parties to the call. A call may be assigned to a first time slot on a first T1, between the customer and the original switch then routed to a second time slot on a second T1 by a first switch. A second switch connected to the first switch by the second T1 may route the call to a third switch on a third time slot on a third T1. A call experiences as many switching stages as are necessary to route a call to its destination. The T1s interconnecting the switches may also use out-of-band signaling, providing a separate path as a signaling channel to communicate control information between the switches.
The end office switch 20 includes a plurality of digital interface units 30 ("DIUs"), also known as "T1 interface units," in communication with a switch matrix 25, such as a time division switched network (TDSN). One DIU 30 interconnects a plurality of T1s to the TDSN. For example, in the 4ESS switch available from Lucent Technology, Inc., a DIU 30 interfaces with as many as five T1's. Some of the T1's may connect to customers as described above, others may connect to other switches within the network. In the 4ESS, the output of the DIU is another trunk carrying 120 multiplexed time slots. The 120 multiplexed time slots are input to the TDSN.
The switch 20 contains a switch signaling interface 40 ("SSI"). The SSI 40 is a hardware element that permits the switch 20 to monitor and respond to control signals contained within the D channel of the trunks 10 connected to the switch 20. The switch 20 routes the D channels of the T1's input to the DIU 30 to a single SSI 40. Each SSI 40 includes a T1 facility access 41 (T1FA), three node processors 42, and six line interfaces 43. The SSI is organized as three packets, each including a pair of line interfaces 43 and one node processor 42, that serve up to eight customers. A failure of the T1FA causes the switch 20 to be isolated from the 24 D channels of various trunks; failure of a packet causes the switch to lose track of eight D channels. Alternatively, the switch 20 may interface with a high density switch signaling interface("HD") 50 as a substitute for the SSI 40. A particular switch 20 may contain more than one SSI 40 or HD 50 (not shown).
The switch 20 can switch any time slot from any T1 to any time slot of any other T1. However, the switch 20 establishes a dedicated pathway that routes the D channels of the T1's to the SSI 40. As traffic on a single T1 10 enters the switch 20, the DIU 30 multiplexes the channels of the T1 with channels from the other T1's connected to the DIU 30. The DIU 30 generates an output carrying the multiplexed output of the several T1's. The switch 20 switches the D channels contained in the DIU output to the SSI 40. Also, D channels from other DIUs 30 may be switched to the same SSI 40. A control processor (not shown) interfaces with the SSI 40 to monitor the status of the D channels and execute commands contained therein.
The architecture of these known switches suffer from a disadvantage because SSI maintenance disables the trunks that the SSI 40 services. If the switch 20 is an end office switch, SSI maintenance causes an interruption in service to the customer. During maintenance, all or part of the SSI 40 is disabled, severing the connection between the SSI and the D channels and isolating the D channels from the control processor. Depending upon the type of maintenance, either an individual packet or the T1FA 41 is disabled, disconnecting from eight to twenty-four customers. Because customers expect and demand continuous, uninterrupted communication service, a communication service provider may notify and negotiate maintenance times with its customers. Using the current architecture, the provider must negotiate a single time when up to twenty-four customers consent to termination of service. Customers rarely entertain such negotiations with good humor.
Accordingly, there is a need in the art for a switching architecture that permits SSI maintenance without interruption of customer service.